U.S. patent application number 16/064751 was filed with the patent office on 2019-01-10 for hollow core optical fiber and a laser system.
This patent application is currently assigned to NKT Photonics A/S. The applicant listed for this patent is NKT Photonics A/S. Invention is credited to Christian JAKOBSEN, Jens Kristian LYNGSOE, Mattia MICHIELETTO.
Application Number | 20190011634 16/064751 |
Document ID | / |
Family ID | 59089071 |
Filed Date | 2019-01-10 |
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United States Patent
Application |
20190011634 |
Kind Code |
A1 |
LYNGSOE; Jens Kristian ; et
al. |
January 10, 2019 |
HOLLOW CORE OPTICAL FIBER AND A LASER SYSTEM
Abstract
A hollow core photonic crystal fiber (PCF) comprising an outer
cladding region and 7 hollow tubes surrounded by the outer cladding
region. Each of the hollow tubes is fused to the outer cladding to
form a ring defining an inner cladding region and a hollow core
region surrounded by the inner cladding region. The hollow tubes
are not touching each other, but are arranged with distance to
adjacent hollow tubes. The hollow tubes each have an average outer
diameter d2 and an average inner diameter d1, wherein d1/d2 is
equal to or larger than about 0.8, such as equal to or larger than
about 0.85, such as equal to or larger than about 0.9. Also a laser
system is disclosed.
Inventors: |
LYNGSOE; Jens Kristian;
(Hornb.ae butted.k, DK) ; JAKOBSEN; Christian;
(Virum, DK) ; MICHIELETTO; Mattia; (Copenhagen NV,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NKT Photonics A/S |
Birkerod |
|
DK |
|
|
Assignee: |
NKT Photonics A/S
Birkerod
DK
|
Family ID: |
59089071 |
Appl. No.: |
16/064751 |
Filed: |
December 22, 2016 |
PCT Filed: |
December 22, 2016 |
PCT NO: |
PCT/DK2016/050460 |
371 Date: |
June 21, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/02328 20130101;
C03B 2203/42 20130101; G02B 6/4296 20130101; G02B 6/02366 20130101;
C03B 2203/16 20130101; C03B 2203/14 20130101; G02B 6/02338
20130101; G02B 6/02371 20130101; G02B 6/4206 20130101; C03B 37/0122
20130101 |
International
Class: |
G02B 6/02 20060101
G02B006/02; G02B 6/42 20060101 G02B006/42 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2015 |
DK |
PA 2015 70877 |
Apr 27, 2016 |
DK |
PA 2016 70263 |
Claims
1. A hollow core photonic crystal fiber (PCF) comprising an outer
cladding region and 7 hollow tubes surrounded by said outer
cladding region, wherein each of said hollow tubes is fused to said
outer cladding region to form a ring defining an inner cladding
region and a hollow core region surrounded by said inner cladding
region, wherein said hollow tubes are not touching each other.
2. The hollow core PCF of claim 1, wherein said hollow tubes each
have an average outer diameter d2 and an average inner diameter d1,
wherein d1/d2 is equal to or larger than about 0.8, such as equal
to or larger than about 0.85, such as equal to or larger than about
0.9.
3. The hollow core PCF of claim 1 or claim 2, wherein said hollow
tubes have a center to center distance .LAMBDA. between adjacent
hollow tubes which is between about 1.01*d2 and about 1.5*d2, such
as between 1.05*d2 and 1.2*d2.
4. The hollow core PCF of any one of the preceding claims, wherein
said hollow tubes have substantially parallel center axes.
5. The hollow core PCF of any one of the preceding claims, wherein
said hollow core region has a core diameter D of from about 10
.mu.m to about 100 .mu.m, such as from about 10 .mu.m to about 60
.mu.m, such as from about 20 .mu.m to about 50 .mu.m, such as from
about 25 .mu.m to about 40 .mu.m.
6. The hollow core PCF of claim 5, wherein said average outer tube
diameter relative to the core diameter d2/D is from about 0.5 to
about 0.75, such as from about 0.65 to about 0.72.
7. The hollow core PCF of any one of the preceding claims, wherein
each of said hollow tubes comprises a core center facing region
with a wall thickness t, wherein said wall thickness is up to about
2.1 .mu.m, such as up to about 1 .mu.m, such as in the range from
about 150 to about 350 nm or in the range from about 650 to about
850 nm or in the range from about 900 to about 2.1 .mu.m.
8. The hollow core PCF of claim 7, wherein said wall thickness t of
each of said hollow tubes is substantially identical, preferably
said hollow tubes are substantially identical and are arranged with
identical distance to adjacent hollow tubes.
9. The hollow core PCF of claim 7, wherein at least one of the
hollow tubes has a different wall thickness t than at least one
other of the hollow tubes, preferably 3 of the hollow tubes have a
wall thickness and the remaining 4 hollow tubes have another wall
thickness.
10. The hollow core PCF of claim 9, wherein at least one of the
hollow tubes has a wall thickness which is at least about 5% larger
than the wall thickness of at least one other of the hollow tubes,
preferably at least one of the hollow tubes has a wall thickness
which is at least about 10% larger than the wall thickness of at
least one other of the hollow tubes.
11. The hollow core PCF of any one of the preceding claims, wherein
each of said hollow tubes is substantially circular.
12. The hollow core PCF of any one of the preceding claims, wherein
each of said hollow tubes has a long inner diameter D.sub.l and a
short inner diameter D.sub.s perpendicular to the long inner
diameter D.sub.l, wherein D.sub.l is determined in radial
direction.
13. The hollow core PCF of any one of the preceding claims, wherein
at least one of said hollow tubes comprises a nested sub tube
arranged in the hollow structure of said hollow tube and fused to
said hollow tube, said sub tube has an average outer diameter
d.sub.sub, which is substantially smaller than said average inner
diameter d of said hollow tube, said average outer diameter
d2.sub.sub is preferably up to about 0.9*d2 of said hollow tube,
such as up to about 0.9*d2, preferably said internal sub tube is
fused to said hollow tube at its maximal radial distance to the
core center axis.
14. The hollow core PCF of any one of the preceding claims, wherein
at least one of said hollow tubes comprises one or more nodules
arranged at a core center facing region of one or more of the
hollow tubes, preferably said nodules are arranged at a boundary of
the hollow core region, said nodules are preferably arranged to be
anti-resonant at an operating wavelength, so that light of a
fundamental mode is substantially excluded from the nodules.
15. The hollow core PCF of any one of the preceding claims, wherein
the minimum distance between adjacent hollow tubes is at least
about 0.1 .mu.m, such as at least about 1 .mu.m, such as at least
about 2 .mu.m, such as at least about 5 .mu.m.
16. The hollow core PCF of any one of the preceding claims, wherein
the hollow core region and said hollow tubes independently of each
other comprise gas selected from air, argon, nitrogen or mixtures
comprising any of the mentioned gasses, optionally said hollow core
region and said hollow tubes independently of each other are
vacummated or filled with pressurized gas.
17. The hollow core PCF of any one of the preceding claims, wherein
the outer cladding region has an outer diameter of at least about
125 .mu.m, 150 .mu.m, such as at least about 200 .mu.m.
18. The hollow core PCF of any one of the preceding claims, wherein
the outer cladding region and/or the hollow tubes comprise a solid
glass material, preferably said solid material is of silica,
optionally doped with refractive index modifying dopant.
19. The hollow core PCF of any one of the preceding claims, wherein
the outer cladding region comprises a photonic bandgap structure
surrounding said inner cladding region.
20. The hollow core PCF of claim 19, wherein the outer cladding
region comprises a outer background material having a refractive
index N.sub.oc and a plurality of inclusions having a refractive
index different from the refractive index of the background
material, said plurality of inclusions in the outer background
material is preferably arranged in a cross-sectional pattern
comprising at least two rings of inclusions surrounding the inner
cladding region, such as at least 3 rings, such as at least 4 rings
of inclusions.
21. The hollow core PCF of claim 20, wherein the plurality of
inclusions in the outer background material is arranged in a
substantially hexagonal pattern.
22. The hollow core PCF of claim 20 or claim 21, wherein the
plurality of inclusions are of solid material, such as down doped
solid material e.g. doped with fluorine and/or boron.
23. The hollow core PCF of claim 20 or claim 21, wherein the
plurality of inclusions are voids or of gas, such as air.
24. The hollow core PCF of any one of claims 20-23, wherein said
plurality of inclusions have an average diameter (d.sub.inc) of up
to about 2.5 .mu.m, such as up to about 2 .mu.m, such as between
about 1.1 .mu.m and 1.8 .mu.m, such as between about 1.15 .mu.m and
about 1.7 .mu.m, such as between about 1.2 .mu.m and about 1.5
.mu.m, such as about 1.3 .mu.m.
25. The hollow core PCF of any one of claims 20-24, wherein said
plurality of inclusions are arranged at a pitch (.LAMBDA..sub.in)
of up to about 6 .mu.m, such as up to about 5 .mu.m, such as up to
about 4 .mu.m, such as between about 2 .mu.m and 4 .mu.m.
26. A laser system for delivering laser light to a user apparatus,
said laser system comprising a laser light source and a fiber
delivery cable for delivering light from the laser light source to
the user apparatus, wherein said fiber delivery cable comprises a
hollow core PCF according to any one of the preceding claims.
27. The laser system of claim 18, wherein said laser light source
is configured for generating laser light pulses and is optically
connected to said fiber delivery cable, preferably said laser light
source is a femtosecond laser source.
28. The laser system of claim 47, wherein said laser light source
has a pump duration of from about 30 fs to about 30 ps, such as
from about 100 fs to about 10 ps.
29. The laser system of claim 47 or claim 48, wherein said laser
light source has a peak power determined at the exit of the laser
light source which is at least about 5 kW, such as at least about
10 kW, such as at least about 30 kW, such as at least about 50
kW.
30. The laser system of any one of claims 47-49, wherein said laser
light source is a mode locked laser, such as an actively mode
locked laser or a passively mode locked laser, said mode locked
laser preferably comprises one or more amplifiers.
31. The laser system of any one of claims 45-50, wherein said
hollow core PCF is configured for single mode guiding of at least
one wavelength in the range from about 200 nm to about 4.5 .mu.m,
preferably at least one wavelength in the range from 1000 nm to
about 1100 nm.
32. The laser system of claim 51, wherein said hollow core PCF is
configured for guiding a continuum of light wavelengths, preferably
spanning over at least about 0.1 .mu.m, such as at least about 0.3
.mu.m, such as at least about 0.5 .mu.m.
33. The laser system of any one of claims 26-32, wherein said
hollow core PCF has a first fiber end which is adapted for being
connected to said user apparatus and a second fiber end optically
connected to an output fiber of said laser light source via a fiber
coupling structure.
34. The laser system of claim 20, wherein said fiber coupling
structure comprises at least one of a focusing lens, a graded-index
element (GRIN), a protection element, or a ferrule structure.
35. The laser system of claim 33 or claim 34, wherein said first
fiber end fiber is mounted in a ferrule structure.
Description
TECHNICAL FIELD
[0001] The invention relates to a hollow core photonic crystal
fiber and a laser system comprising such hollow core optical
fiber.
BACKGROUND ART
[0002] Hollow core photonic crystal fibers (PCF) have been known
for many years and in some applications are very attractive to use
compared to solid core PCF due to their very low propagation loss,
low nonlinear effects and high damage threshold.
[0003] A hollow-core fiber is an optical fiber which guides light
essentially within a hollow region, so that only a minor portion of
the optical power propagates in solid fiber material surrounding
the core. The hollow region may be filled with air or any other gas
or it may be evacuated to have a very low gas pressure.
[0004] Over the years several types of hollow core PCFs have been
developed, such as hollow-core photonic bandgap fibers e.g. as
described in U.S. Pat. No. 6,892,018, Kagome design type fibers
e.g. as described in U.S. Pat. No. 8,306,379.
[0005] The kagome type fiber guides light by means of an
anti-resonant effect and allows for a substantially broader
spectral transmission than that achieved in photonic bandgap
fibers.
[0006] In recent years such hollow core anti-resonant fibers (ARFs)
have been studied intensively in particular with the aim of further
improving the fiber with respect to low loss and broad spectral
transmission.
[0007] In an attempt to reduce the attenuation Kolyadin et al,
Optics Express, Vol 21, pages 9514 to 9519, 2013 suggested a hollow
core fiber with a hollow core surrounded by eight identical
capillaries. It was concluded that to reduce loss, a larger core
size is preferred and by increasing the number of capillaries in
the cladding a reduced loss could be achieved.
[0008] Another drawback of prior art hollow core anti-resonant
fibers is that they are not purely single mode waveguides because
they also support some higher-order modes (HOMs). Since these HOMs
often have a relatively low loss, it is very difficult to launch a
pure LP01 mode without HOM contamination and further HOMs can be
excited by bending or external stress.
[0009] In an attempt to solve this problem and to achieving the
highest possible suppression of HOMs while maintaining a reasonably
low loss for the LP01 mode, a modified fiber design was suggest in
"Broad-band robustly single-mode hollow-core PCF by resonant
filtering of higher order modes" by Gunedi et al., Cornell
university library arXiv:1508.06747 [physics.optics], 27 Aug. 2015.
The ARR fiber described in this article comprises a central hollow
core (inner diameter D) surrounded by six evenly spaced and
non-touching capillaries (in the following also called hollow
tubes) with a wall thickness t and an inner diameter d, supported
within a thick-walled supporting capillary. The non-touching glass
elements had modes which were tailored to ensure resonant
phase-matched coupling to higher-order core modes, causing them to
leak at a very high rate into the supporting solid glass sheath. It
was found that extremely high suppression of HOMs could be achieved
over broad wavelength bands where the fiber guides with a low loss,
provided the ratio d/D.apprxeq.0.68 and the walls of the tubes
walls T were thin enough i.e. t/D=0.01.
[0010] WO15185761 describes a similar anti-resonant hollow-core
fiber comprising a first tubular, cladding element which defines an
internal cladding surface, a plurality of second tubular elements
which are attached to the cladding surface and together define a
core with an effective radius, the second tubular elements being
arranged in spaced relation and adjacent one of the second tubular
elements having a spacing there between, and a plurality of third
tubular elements, each nested within a respective one of the second
tubular elements. It was concluded that the optimal number of tubes
for suppressing HOMs was six.
DESCRIPTION OF THE INVENTION
[0011] The object of the invention is to provide a hollow core PCF
which alleviates at least one of the drawbacks described above.
[0012] In an embodiment it is an object to provide a hollow core
PCF which has a high spectral transmission and a high suppression
of HOMs.
[0013] In an embodiment it is an object to provide a hollow core
PCF which has a high single mode transmission efficiency and is
substantially insensitive to bending.
[0014] In an embodiment it is an object to provide a hollow core
PCF which in a simple way can be designed to have a desired low
loss transmission band with a bandwidth of at least about 50 nm and
preferably with a transmission loss of less than about 50
dB/Km.
[0015] In an embodiment it is an object to provide a hollow core
PCF which has a transmission band comprising wavelengths in the
range of from about 400 nm to about 1200 nm.
[0016] These and other objects have been solved by the invention or
embodiments thereof as defined in the claims and as described
herein below.
[0017] It has been found that the invention or embodiments thereof
have a number of additional advantages which will be clear to the
skilled person from the following description.
[0018] The phrase "radial distance" means distance determined in
radial direction from the center axis of the hollow core. The
phrase "radial direction" is a direction from the core center axis
and radially outwards.
[0019] The term "substantially" should herein be taken to mean that
ordinary product variances and tolerances are comprised.
[0020] The term "about" is generally used to include what is within
measurement uncertainties. The term "about" when used in ranges
should herein be taken to mean that what is within measurement
uncertainties are included in the range.
[0021] It should be emphasized that the term "comprises/comprising"
when used herein is to be interpreted as an open term, i.e. it
should be taken to specify the presence of specifically stated
feature(s), such as element(s), unit(s), integer(s), step(s)
component(s) and combination(s) thereof, but does not preclude the
presence or addition of one or more other stated features.
[0022] Throughout the description or claims, the singular
encompasses the plural unless otherwise specified or required by
the context.
[0023] Diameters, thickness and other structural values are seen in
a cross sectional view of the hollow core PCF unless otherwise
specified or clear from the context.
[0024] The hollow core photonic crystal fiber (PCF) of the
invention comprises an outer cladding region and 7 hollow tubes
surrounded by the outer cladding region. Each of the hollow tubes
is fused to the outer cladding to form a ring defining an inner
cladding region and a hollow core region surrounded by the inner
cladding region. The hollow tubes are not touching each other. It
has been found to be advantageous for suppressing of undesired
higher order modes that the hollow tubes each has an average outer
diameter which is referred to as d2 and an average inner diameter
which is referred to as d1. The ration d1/d2 is equal to or larger
than about 0.8.
[0025] The average outer diameter d2 and/or the average inner
diameter of the respective hollow tubes may be equal or different
from hollow tube to hollow tube
[0026] In an embodiment d1 and/or d2 is/are substantially identical
for at least three of the hollow tube, such as for all of the seven
hollow tubes.
[0027] According to the invention and against all teaching of prior
art hollow core PCF it has surprisingly been found that a hollow
core PCF with a non-touching inner cladding structure where there
are seven non-touching tubes provides a highly increased
suppression of higher order modes (HOMs) than the above described
hollow core PCF with six 6 non-touching inner cladding tubes. In
particular--and as shown in the example--it has been found that the
hollow core PCF of the invention with seven non-touching tubes has
an increased suppression of HOMs with azimuthal number above 1,
such a third order HOMs while simultaneously having low loss in the
fundamental mode of at least a transmission band below 2 .mu.m,
preferably below 1.5 .mu.m.
[0028] The term "transmission band" is herein used to mean a band
with a bandwidth of at least about 10 nm, such as at least about 25
nm and preferably at least about 50 nm or even at least about 100
nm of transmission wavelength.
[0029] The term "low loss transmission band" means a transmission
band for light in its fundamental mode with a transmission loss of
less than about 100 dB/km, preferably less than about 60 dB/km and
more preferably less than about 50 dB/km.
[0030] It has further been found that by increasing the ration
d1/d2 the low loss transmission band may be achieved at even lower
wavelengths such as a low loss transmission band comprising
wavelengths below 1200 nm, such as below 1 .mu.m, such as below 800
.mu.m, such as below 600 my. Advantageously the ration d1/d2 is
equal to or larger than about 0.85, such as equal to or larger than
about 0.9.
[0031] The outer cladding comprises solid cladding material and is
preferably entirely of solid material.
[0032] In an embodiment the hollow tubes are arranged with a
substantially equal distance to adjacent hollow tubes. Thereby a
very high Gaussian beam quality may be obtained. In an alternative
embodiment the hollow tubes are arranged with different distances
to adjacent hollow tubes. In the latter embodiment the variation in
distance between adjacent hollow tubes is provided by having
different outer diameters d2 of the tubes. Where the inner diameter
d1 and/or the outer diameter d2 of the hollow tubes differs, the
inner diameter d1, respectively the outer diameter d2 is calculated
as the average d1 and d2 values, unless the calculation is the
hollow tube thickness or otherwise specified. Accordingly the
ration d1/d2 is determined by the average d1 and d2 values. It has
been found that the optimal suppression of HOMs requires that the
minimum distance between adjacent hollow tubes is relatively low
and preferably less than about half the outer diameter d2 of the
hollow tubes. On the other hand, the distance should not be too
narrow because this may result in the risk of fully eliminating the
distance along parts of or along the whole fiber length due to
surface tension and material attraction during drawing of the
fiber. To avoid such risk it is desired that the minimum distance
between adjacent hollow tubes is at least 0.01 times d2.
[0033] Advantageously the minimum distance between adjacent hollow
tubes is at least about 0.1 .mu.m, such as at least about 1 .mu.m,
such as at least about 2 .mu.m, such as at least about 5 .mu.m.
[0034] In an embodiment the minimum distance between adjacent
hollow tubes is about 5 .mu.m or less, such as about 4 .mu.m or
less.
[0035] It has been found to be optimal that the center to center
distance .LAMBDA. between adjacent hollow tubes is between about
1.01*d2 and about 1.5*d2, such as between 1.05*d2 and 1.2*d2.
[0036] The hollow tubes advantageously have substantially parallel
center axis. In practice the center axis of the hollow tubes may
deviate slightly from being straight due to process variations. The
hollow tubes may for example be helical around the core with a very
long pitch, such as a pitch of up to 1 km, such as from 1 cm to 100
m.
[0037] Due to the structure of the hollow core PCF the core may in
principle be designed with any diameter of the core region. The
core region is advantageously substantially circular. In an
embodiment the core region is not circular, but rather oval or
angular, such as substantially pentagonal. The core diameter D is
defined as the diameter of the largest circle inscribed by the 7
hollow tubes.
[0038] Advantageously the hollow core region has a core diameter D
of from about 10 .mu.m to about 100 .mu.m, such as from about 10
.mu.m to about 60 .mu.m.
[0039] It has been found that the optimal core diameter D is
scalable with the central wavelength of the transmission band of
the hollow core PCF.
[0040] For a central wavelength of about 1.0 .mu.m. the core
diameter D is advantageously as from about 20 .mu.m to about 50
.mu.m, such as from about 25 .mu.m to about 40 .mu.m. The preferred
core diameter D is directly proportionally scaled with the central
wavelength of the transmission band of the hollow core PCF.
[0041] It has been found that the hollow core PCF may have a very
high beam quality even where the core is relatively large. In an
embodiment the beam quality M.sup.2 is about 1.75 or less, such as
about 1.6 or less, such as about 1.5 or even less.
[0042] In an embodiment the average outer tube diameter relative to
the core diameter d2/D is from about 0.5 to about 0.75, such as
from about 0.65 to about 0.72, thereby a desired minimum distance
between the hollow tubes and a very effective suppression of
HOMs--in particular of 3.sup.rd and 4.sup.th HOMs--are ensured.
[0043] Further the mode quality of the beam obtained with a hollow
core PCF average outer tube diameter relative to the core diameter
d2/D is from about 0.5 to about 0.75 has shown to be very high.
[0044] It has been found the wall thickness of the hollow tubes
largely influences the low loss transmission band or bands of the
hollow core PCF. In fact, the wall thickness t of relevance has
been found to be the core center facing region of the wall of the
hollow tubes.
[0045] Thus it has been found that the wall thickness t (or the
average value of t) of the core center facing region of the hollow
tubes mainly affects the location with respect to wavelengths of
the low loss transmission bands. In general the hollow core PCF may
have one or several low loss transmission bands, such as 1, 2, 3, 4
or even further low loss transmission bands.
[0046] Advantageously the hollow core PCF has at least 3 low loss
transmission bands, such as at least 4 low loss transmission bands.
For obtaining one or more low loss bands below 1.2 .mu.m it is
desired that the wall thickness t is up to about 2.1 .mu.m, such as
up to about 1 .mu.m, such as in the range from about 150 to about
350 nm or in the range from about 650 to about 850 nm or in the
range from about 900 to about 2.1 .mu.m.
[0047] For example it has been found that for a hollow core PCF of
an embodiment of the invention the fiber has a low loss
transmission at 1030-1064 nm for a wall thickness t=150-350 nm,
preferably 200-300 nm (low loss transmission band with highest
wavelengths--band I), for a wall thickness t=650-850 nm, preferably
700-800 nm (2nd highest wavelengths low loss transmission
band--band II), and for a wall thickness of 5 times first range for
a third transmission band comprising 1030-1064 nm i.e. 5*150 nm to
5*350 nm, seven times first range for a fourth transmission band
comprising 1030-1064 nm and so on).
[0048] Advantageously the hollow core PCF is designed such that the
hollow core guides light in the fundamental mode and comprises at
least one wavelength between 200 nm and 4000 nm wavelength, such as
between 400 and 2000 nm, such as between 800 and 1600 nm, such as
between 1000 and 1100 nm, where the loss is less than about 1000
dB/km.
[0049] Advantageously the hollow core PCF has a low loss
transmission band for the fundamental mode with a transmission loss
of less than about 100 dB/km, preferably less than about less than
60 dB/km and more preferably less than 50 dB/km comprising
wavelengths in the interval between 1000 and 1100 nm and preferably
comprising at least the wavelengths 1030-1064 nm
[0050] In an embodiment the hollow core PCF has a transmission loss
for HOMs in its low transmission band for the fundamental mode of
larger than about 2000 dB/km, thereby ensuring an effective single
mode transmission.
[0051] It has further been found that the hollow core PCF is
substantially insensitive to bending, for example it has been found
that the hollow core PCF of an embodiment of the invention has a
low loss which is less than about 5%/km at 1030-1060 nm when coiled
at a diameter of about 6 cm.
[0052] Advantageously the wall thickness t of each of the hollow
tubes is substantially identical. Preferably the hollow tubes are
substantially identical and are arranged with identical distance to
adjacent hollow tubes. Thereby the hollow core PCF becomes simpler
to design for a desired wavelength transmission.
[0053] In an embodiment at least one of the tubes has a different
wall thickness t than at least one other of the hollow tubes.
Preferably 3 of the hollow tubes have a wall thickness and the
remaining 4 hollow tubes have another wall thickness. It has been
found that by this arrangement the hollow core PCF may become
birefringent, largely depending on the difference in wall thickness
t and the relative position of the hollow tubes with larger and
smaller wall thickness t.
[0054] In an embodiment at least one of the hollow tubes has a wall
thickness which is at least about 5% larger than the wall thickness
of at least one of the other hollow tubes, preferably at least one
of the hollow tubes has a wall thickness which is at least about
10% larger than the wall thickness of at least one other of the
hollow tubes.
[0055] In an embodiment each of the hollow tubes is substantially
circular. In practice it is very difficult to obtain absolutely
circular hollow tubes, first of all due to a slight deformation
where the respective hollow tubes are fused to the inner side of
the outer cladding and secondly because the hollow tubes may be
attracted to adjacent hollow tubes during the drawing of the
fiber.
[0056] In an embodiment each of the hollow tubes has a long inner
diameter D.sub.l and a short inner diameter D.sub.s perpendicular
to the long inner diameter D.sub.l, wherein D.sub.l is determined
in radial direction from the hollow core center axis.
[0057] Advantageously the ration D.sub.s/D.sub.l is from about 0.5
to about 0.99, such as from about 0.8 to about 0.99.
[0058] In an embodiment the D.sub.s/D.sub.l ratio is larger than
about 0.9, such as larger than about 0.95 which in practice is
simpler to produce.
[0059] In an embodiment the D.sub.s/D.sub.l ratio is smaller than
about 0.95, such as smaller than 0.9 which results in a relative
large core diameter D and thereby may add to reduce transmission
loss for the fundamental mode.
[0060] In an embodiment at least one of the hollow tubes comprises
at least one nested sub tube arranged in the hollow structure of
the hollow tube wherein the sub tube is fused to the hollow tube,
preferably aligned with the fusion to the outer cladding i.e.
farthest from the core region.
[0061] The hollow tubes with one or more nested tubes may for
example be as described for the PCF of WO15185761 comprising second
tubular elements with third tubular elements nested within
respective second tubular elements, with the difference that the
hollow core PCF has 7 second tubular elements with nested third
tubular tubes.
[0062] The sub tube advantageously has an average outer diameter
d.sub.sub, which is substantially smaller than the average inner
diameter d of the hollow tube. The average outer diameter
d2.sub.sub is preferably up to about 0.9*d2 of the hollow tube,
such as up to about 0.9*d2, preferably the internal sub tube is
fused to the hollow tube at its maximal radial distance to the core
center axis.
[0063] In an embodiment at least one of the hollow tubes comprises
one or more nodules arranged at a core center facing region of one
or more of the hollow tubes, preferably the nodules are arranged at
a boundary of the hollow core region, the nodules are preferably
arranged to be anti-resonant at an operating wavelength, so that
light of a fundamental mode is substantially excluded from the
nodules. Thereby an even stronger confinement of the fundamental
mode may be obtained to further reduce loss of light in the
fundamental mode transmitted in the core region of the hollow core
PCF.
[0064] The nodules may advantageously be in form of nodule-like, or
bead-like, formation or a locally thicker region of the wall
extending along at least a part of the length of the fiber. Such
nodules are sometimes referred to as nodes or blobbies in the prior
art optical fibers e.g. as described in U.S. Pat. No. 7,321,712.
The nodules and the remaining parts of the hollow tubes
advantageously are of identical material.
[0065] The core region and/or the hollow tubes may in principle
comprise any fluid. Preferably the core region and/or the hollow
tubes independently of each other comprise gas determined at
standard ambient temperature and pressure (SATP) as a temperature
of 25.degree. C. and an absolute pressure of exactly 100 000 Pa.
Suitable gasses include air, argon, nitrogen or mixtures comprising
any of the mentioned gasses. Optionally the hollow core region
and/or the hollow tubes independently of each other are vacummated
(evacuated to have a very low gas pressure) or filled with
pressurized gas.
[0066] In an embodiment the hollow core region and/or the hollow
tubes are vacummated to a pressure of about 1 mbar or less, such as
to a pressure of about 0.1 mbar or less, such as to a pressure of
about of 0.01 mbar or less at standard temperature.
[0067] In an embodiment the hollow core region and/or the hollow
tubes are pressurized to a pressure of up to 2 bars, such as up to
about 1.5 bars at standard temperature.
[0068] The outer cladding region may in principle have any size
provided that it provides the hollow core PCF with a sufficient
mechanical support for the hollow tubes. In an embodiment the outer
cladding region has an outer diameter of at least about 125 .mu.m,
150 .mu.m, such as at least about 200 .mu.m.
[0069] In general it is desired that the hollow core PCF is of one
single material which is preferably glass, more preferably silica,
optionally doped with a refractive index modifying dopant. In an
embodiment one or more, such as all of the hollow tubes are of
doped silica and the outer cladding region is of undoped silica.
The dopant may for example include index changing materials such as
F, Ge, P, B or combinations thereof.
[0070] Further it has been found that confinement loss may be
reduced by providing the outer cladding region to comprise a
photonic bandgap structure surrounding said inner cladding
region.
[0071] The photonic bandgap structure may be provided by any means,
e.g. by providing the outer cladding region with an index grating
comprising concentric rings with different refractive index and/or
by including microstructures having different index.
[0072] In an embodiment the outer cladding region comprises an
outer background material having a refractive index N.sub.oc and a
plurality of inclusions having a refractive index different from
the refractive index of the background material. The inclusions
advantageously have a lower refractive index than the refractive
index of the background material. The inclusions are preferably
extending substantially parallel with the core region. The
inclusions may extend in a length section of the hollow core PCF or
in substantially the entire length of the hollow core PCF.
[0073] In an embodiment the plurality of inclusions in the outer
background material is arranged in a cross-sectional pattern
comprising at least two rings of inclusions surrounding the inner
cladding region, such as at least 3 rings, such as at least 4 rings
of inclusions.
[0074] The phrase "ring of inclusions" refers to inclusions of the
outer cladding inclusions having substantially equal radial
distance to the core and being aligned in a ring configuration
surrounding the core. Typically, a ring of inclusions is not fully
circular, but rather is shaped with a number of soft angles, such
as in a hexagonal shape. Preferably all the inclusions of a ring of
inclusions are of substantially the same size and preferably of
same solid material, voids and/or gas.
[0075] The background material may advantageously be silica, such
as un-doped silica or doped silica. The dopant may for example
include index changing materials such as F, Ge, P, B or
combinations thereof.
[0076] In an embodiment the plurality of inclusions in the outer
background material is arranged in a substantially hexagonal
pattern.
[0077] In an embodiment the plurality of inclusions are voids or of
gas, such as air.
[0078] The diameter(s) of the inclusions is/are advantageously
selected to minimize loss of a selected wavelength or a range of
wavelengths. Thus the diameter(s) may be selected to optimize for a
desired transmission profile.
[0079] In an embodiment the inclusions have substantially identical
diameters.
[0080] Advantageously the plurality of inclusions have an average
diameter (d.sub.inc) of up to about 2.5 .mu.m, such as up to about
2 .mu.m, such as between about 1.1 .mu.m and 1.8 .mu.m, such as
between about 1.15 .mu.m and about 1.7 .mu.m, such as between about
1.2 .mu.m and about 1.5 .mu.m, such as about 1.3 .mu.m.
[0081] Also the distance(s) between the respective inclusions has
shown to be relevant for optimizing (minimizing) confinement loss.
In an embodiment for optimizing confinement loss the plurality of
inclusions are arranged at a pitch (.LAMBDA..sub.inc) of up to
about 6 .mu.m, such as up to about 5 .mu.m, such as up to about 4
.mu.m, such as between about 2 .mu.m and 4 .mu.m.
[0082] In an embodiment the inclusions are arranged at a pitch
(.LAMBDA..sub.inc) of up to about 3.5 .mu.m, such as up to about 3
.mu.m, such as up to about 2.5 .mu.m, such as between about 1.1
.mu.m and 2 .mu.m.
[0083] The invention also comprises a laser system for delivering
laser light to a user apparatus, the laser system comprising a
laser light source and a fiber delivery cable for delivering light
from the laser light source to the user apparatus, wherein the
fiber delivery cable comprises a hollow core PCF as described
above.
[0084] The fiber delivery cable comprising the hollow core PCF may
preferably have a length of up to 50 m, such as from about 0.3 m to
about 20 m, such as from about 1 m to about 15 m.
[0085] Advantageously the laser light source is configured for
generating laser light pulses and is optically connected to the
fiber delivery cable, preferably the laser light source is a
femtosecond laser source.
[0086] The laser light source may in an embodiment be arranged for
directly feed the light to the hollow core PCF e.g. by being fused
to the hollow core PCF. In an embodiment the laser light source is
arranged for feeding the light to the hollow core PCF via one or
more optical elements and/or via free space.
[0087] In an embodiment the laser light source has a pump duration
of from about 30 fs to about 30 ps, such as from about 100 fs to
about 10 ps.
[0088] In an embodiment the laser light source has a peak power
determined at the exit of the laser light source which is at least
about 5 kW, such as at least about 10 kW, such as at least about 30
kW, such as at least about 50 kW.
[0089] The laser light source is advantageously a mode-locked laser
light source. In an embodiment laser light source is an actively
mode locked laser. In an embodiment the laser light source is a
passively mode locked laser. The mode locked laser preferably
comprises one or more amplifiers.
[0090] In an embodiment the hollow core PCF is configured for
guiding light--preferably single mode light--comprising at least
one wavelength in the range from about 200 nm to about 4.5 .mu.m,
preferably at least one wavelength in the range from 1000 nm to
about 1100 nm.
[0091] In an embodiment the hollow core PCF is configured for
guiding a continuum of light wavelengths, preferably spanning over
at least about 0.1 .mu.m, such as at least about 0.3 .mu.m, such as
at least about 0.5 .mu.m.
[0092] In an embodiment the hollow core PCF has a first fiber end
which is adapted for being connected to the user apparatus and a
second fiber end optically connected to an output fiber of the
laser light source via a fiber coupling structure. The fiber
coupling structure preferably provides a protection of the first
fiber end and preferably of a facet of the first fiber end to
ensure that the facet and or the hollow core is not contaminated
with dust, moisture or similar. Further to ensure a safe low loss
connection the coupling structure may comprise a lens, such as a
focusing lens, a graded-index element or other elements such as it
is known in the art for coupling structures for hollow core fibers
in general.
[0093] In an embodiment the fiber coupling structure comprises a
focusing lens, for example such as it is described in U.S. Pat. No.
8,854,728.
[0094] In an embodiment the fiber coupling structure comprises a
graded-index element (GRIN) such as it is described in US
2003/0068150.
[0095] In an embodiment the fiber coupling structure comprises a
protection element such as it is described in U.S. Pat. No.
7,373,062.
[0096] In a preferred embodiment the first fiber end is mounted in
a ferrule structure, preferably with a ferrule structure as
described in PA 2015 70876 DK with the title "PHOTONIC CRYSTAL
FIBER ASSEMBLY" and which application in hereby incorporated by
reference into this disclosure with the proviso that in case of any
inconsistence between the subject matter explicitly disclosed
herein and the incorporated subject matter, the content of the
subject matter explicitly disclosed herein prevail.
[0097] The hollow core PCF as described herein may advantageously
be produced by drawing from a preform wherein the drawing is
performed while controlling the pressure within the hollow tubes
for example as described in U.S. Pat. No. 6,954,574, U.S. Pat. No.
8,215,129, U.S. Pat. No. 7,793,521 and/or in PA 2016 70262 DK with
the title "A RING ELEMENT FOR A PREFORM, A PREFORM AND AN OPTICAL
FIBER DRAWN FROM THE PREFORM"
[0098] The preform is advantageously produced by providing a hollow
tube for the outer cladding region and 7 hollow tubes for the inner
cladding, wherein the hollow tube for the outer cladding region has
an inner diameter which is larger than two times, such as at least
3 times, such as at least 4 times larger than the outer diameter of
the 7 hollow tubes--or if different--of the largest outer diameter
of the 7 hollow tubes for the inner cladding. Preferably the outer
cladding region has an inner diameter which is sufficiently large
to arrange the 7 hollow tubes inside and in contact with the hollow
tube for the outer cladding region such that the 7 hollow tubes are
not touching each other. To arrange the 7 hollow tubes inside the
hollow tube for the outer cladding region a short section of
support elements (e.g. tubes or rods of glass) may be arranged at
each end of the hollow tubes. After having fused the 7 hollow tubes
to the hollow tube for the outer cladding region, the ends of the
fused hollow tubes comprising the support elements may be cut
off.
[0099] The preform may then be drawn in a fiber drawing tower
preferably by simultaneously individual or common pressure control
of the pressure within the respective hollow tubes.
[0100] In an embodiment the pressure control of each of one or more
of the hollow tubes of the preform, also referred to as "elongate
holes" or merely "holes", is provided by arranging a pressure tube
between the hole and a pressure supply. The pressure supply ensures
that the pressure within the hollow tube is controlled to a desired
level via the pressure tube during the drawing of the fiber from
the preform.
[0101] The pressure tube is advantageously a hollow pressure tube.
The phrase "in the tube" refers to in the hollow part of the
pressure tube.
[0102] Thus in an embodiment the method comprises inserting a first
end of a pressure tube into the hole of the preform at a first end
of the preform and subjecting the hole of the preform to a
controlled pressure via the pressure tube during drawing.
Advantageously at least a pressure tube length section comprising
the first end of the pressure tube is inserted into the elongate
hole of the preform. The pressure tube length section should
preferably have a length of at least about 0.5 mm, such as from
about 1 mm to about 20 cm, such as from about 2 mm to about 5 cm,
such as from 0.5 tot cm.
[0103] In practice it is desired that the pressure tube length
section has a length which is sufficient to provide a seal between
the pressure tube length section and the elongate hole and yet it
should not be too long since the length part of the preform
comprising the pressure tube length section may in an embodiment
not be drawn to fiber or if--in an alternative embodiment--it is
drawn to fiber, the resulting fiber will then have different
characteristics than the fiber drawn from preform material without
the tube length section(s)
[0104] It has been found that a safe gas connection may be obtained
between the elongate hole and the pressure tube by simply inserting
the first end of the pressure tube into the hole and optionally
providing that the pressure tube length section is expanded such
that its outer surface fits to the periphery surface of the
elongate hole and/or applying a sealing material, such as glue,
epoxy, grease and/or rubber or any other pliable sealing
material.
[0105] The pressure tube advantageously has an outer diameter and
periphery selected such that it fits into the hole of the preform.
The cross sectional shape of the hole may be round or oval or any
other suitable shape. The surface defining the hole is also
referred to as the periphery surface of the elongate hole. The
pressure tube preferably has an outer cross sectional shape
corresponding to the cross sectional shape of the hole of the
preform, however with an average diameter slightly smaller than the
average diameter of the hole of the preform, such that the first
end of the pressure tube can be inserted into the hole.
[0106] In an embodiment the pressure tube or at least the pressure
tube length section of the pressure tube has an average outer
diameter which is from about 80% up to 100% of an average inner
diameter of the hole, such as from about 90% to about 99% of the
average diameter of the hole.
[0107] Advantageously the pressure tube has a supply section which
is outside the hole i.e. the part of the pressure tube which leads
from the hole to a gas connection to the pressure supply.
[0108] In an embodiment the supply section of the pressure tube has
a supply opening which is in gas connection with a pressure source
for controlling the pressure within the hole.
[0109] In an embodiment the supply section of the pressure tube has
a supply opening which is within a pressure regulating chamber. By
regulating the pressure within the pressure regulating chamber e.g.
by the pressure source, the pressure within the pressure tube is
also regulated and thereby the pressure within the elongate hole is
regulated.
[0110] In an embodiment the supply section of the pressure tube has
a supply opening which is directly connected to a pressure source
for regulating the pressure within the pressure tube and thereby
the pressure within the elongate hole is regulated.
[0111] The pressure tube can in principle be of any material. In an
embodiment the pressure tube is of a thermo-moldable material, e.g.
a material which may be molded or drawn at a fiber drawing tower.
Advantageously the pressure tube is of silica optionally comprising
a polymer coating. In an embodiment at least the supply section of
the pressure tube has an outer polymer coating and optionally the
pressure tube length section is free of polymer coating. The
polymer coating increases the pliability of the pressure tube and
reduces the risk of rupture of the pressure tube.
[0112] By providing the pressure tube of silica with a pressure
tube length section with an uncoated pressure tube length section
and a pressure tube supply section with a polymer coating, a
desirably large hollow part cross-sectional diameter of the
pressure tube may be obtained while at the same time the pressure
tube supply section may be desirably pliable and rupture
resistant.
[0113] As mentioned above several holes of the preform may be
pressure controlled with pressure tubes as described above. The
pressure tubes may be connected to the same pressure supply or to
different pressure supplies.
[0114] All features of the invention and embodiments of the
invention as described above including ranges and preferred ranges
can be combined in various ways within the scope of the invention,
unless there are specific reasons not to combine such features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0115] The above and/or additional objects, features and advantages
of the present invention will be further elucidated by the
following illustrative and non-limiting detailed description of
embodiments of the present invention, with reference to the
appended drawings.
[0116] FIG. 1 shows a cross-section of an embodiment of a hollow
core PCF of the invention where one of the hollow tubes is enlarged
to show the inner diameter d1, the outer diameter d2 and the wall
thickness t.
[0117] FIG. 2 shows a cross-section of an embodiment of a hollow
core PCF of the invention where some of the hollow tubes have a
larger wall thickness t than other of the hollow tubes.
[0118] FIG. 3 shows a cross-section of an embodiment of a hollow
core PCF of the invention where the hollow tubes are oval.
[0119] FIG. 4 shows a cross-section of an embodiment of a hollow
core PCF of the invention where the hollow tubes comprise nested
sub tubes arranged in the hollow structure of hollow tubes and
fused to the hollow tubes.
[0120] FIG. 5 shows a cross-section of an embodiment of a hollow
core PCF of the invention where the hollow tubes comprise nodules
arranged at a core center facing region.
[0121] FIG. 6 shows a cross-section of an embodiment of a hollow
core PCF of the invention where single mode light has been fed to
the hollow core PCF.
[0122] FIG. 7 is a graph showing several low loss transmission
bands of a hollow core PCF of an embodiment of the invention.
[0123] FIG. 8 is a graph showing a broad low loss transmission band
of a hollow core PCF of an embodiment of the invention.
[0124] FIG. 9 is a graph showing the suppression of HOMs of a prior
art hollow core PCF with six hollow tubes.
[0125] FIG. 10 is a graph showing the suppression of HOMs of hollow
core PCF of an embodiment of the invention with seven hollow
tubes.
[0126] FIG. 11 is a schematic drawing of a laser system of an
embodiment of the invention as well as a user apparatus.
[0127] FIG. 12 illustrates a first end of a preform for an
embodiment of a hollow core PCF of the invention with 7 hollow
tubes forming 7 elongate holes and where the first ends of the
respective pressure tubes are inserted into respective holes for
pressure controlling of the holes during drawing.
[0128] FIG. 13a illustrates a hollow core PCF of an embodiment of
the invention comprising an outer cladding region comprising a
photonic band gap structure.
[0129] FIG. 13b is a graph showing the transmission loss for two
variations of the hollow core PCF of the embodiment FIG. 13a.
[0130] FIG. 14 is a graph showing a measurement of the beam quality
M.sup.2 of the fiber fabricated in EXAMPLE 1.
[0131] The figures are schematic and simplified for clarity.
Throughout, the same reference numerals are used for identical or
corresponding parts.
[0132] The hollow core PCF of the invention shown in FIG. 1
comprises an outer cladding region 1 and seven hollow tubes 2
surrounded by said outer cladding region. Each of the hollow tubes
2 is fused at fusing point 3 to the outer cladding 1 to form a ring
defining an inner cladding region and a hollow core region 4
surrounded by the inner cladding region and having a core diameter
D.
[0133] The hollow tubes are not touching each other and are
generally referred to as non-touching hollow tubes. As shown in the
enlargement of a hollow tube, each of the hollow tubes 2 has an
average outer diameter d2 and an average inner diameter d1 and a
wall thickness tin the center facing region of the respective
hollow tubes 2. The outer cladding 1 has an inner diameter ID and
an outer diameter OD. In this embodiment the hollow tubes 2 are
identical and are substantially circular in cross section.
[0134] In the embodiment shown in FIG. 2 the hollow core PCF
comprises an outer cladding region 11 and seven non-touching hollow
tubes 12a, 12b surrounded by said outer cladding region where the
hollow tubes 12a, 12b are fused at fusing point 13 to the outer
cladding 11 to form a ring defining an inner cladding region and a
hollow core region 14 surrounded by the inner cladding region.
Three of the non-touching hollow tubes 12a have a larger wall
thickness t than the remaining four of the non-touching hollow
tubes 12b. As shown each of the respective seven non-touching
hollow tubes 12a, 12b is uniform in thickness, thereby providing a
simpler production of the hollow core PCF.
[0135] In the embodiment shown in FIG. 3 the hollow core PCF
comprises an outer cladding region 21 and seven identical
non-touching hollow tubes 22 surrounded by said outer cladding
region, and the hollow tubes 22 are fused to the outer cladding 21
to form a ring defining an inner cladding region and a hollow core
region 24 surrounded by the inner cladding region. The hollow tubes
22 are oval and each of the hollow tubes 22 has a long inner
diameter D.sub.l and a short inner diameter D.sub.s perpendicular
to the long inner diameter D.sub.l, wherein D.sub.l is determined
in radial direction.
[0136] In the embodiment shown in FIG. 4 the hollow core PCF
comprises an outer cladding region 31 and seven identical
non-touching hollow tubes 32 surrounded by said outer cladding
region, and the hollow tubes 32 are fused at fusing points 33 to
the outer cladding 31 to form a ring defining an inner cladding
region and a hollow core region 34 surrounded by the inner cladding
region. The hollow tubes 32 comprise nested sub tubes 35 arranged
in the hollow structure of the hollow tubes and fused to the hollow
tubes at the fusing points 33 at their respective maximal radial
distance to the core center axis.
[0137] In the embodiment shown in FIG. 5 the hollow core PCF
comprises an outer cladding region 41 and seven non-touching hollow
tubes 42a, 42b surrounded by said outer cladding region where the
hollow tubes 42a, 42b are fused to the outer cladding 41 to form a
ring defining an inner cladding region and a hollow core region 44
surrounded by the inner cladding region. Four of the non-touching
hollow tubes 12a are uniform in thickness of their respective walls
whereas the remaining three of the non-touching hollow tubes 42b
comprise nodules 45 arranged at a core center facing region of the
respective hollow tubes. Since the outer diameter d2 of the
respective non-touching hollow tubes 42a, 42b is substantially
identical, the nodules 45 are arranged at a boundary of the hollow
core region.
[0138] In the embodiment shown in FIG. 6 the hollow core PCF mainly
has a structure as the hollow core PCF of FIG. 1. A not shown
single mode laser source is arranged to laser light to the PCF at
wavelength in a low loss transmission band of the PCF. After 5 m or
even 10 the beam transmitted in the PCF as indicated with ref. 6
has a Gaussian beam quality and is fully single mode. After 10 m of
transmission the loss in the fundamental mode is very low e.g. with
a transmission efficiency of more than 85%, such as more than
90%.
[0139] The graph of FIG. 7 shows several low loss transmission
bands of a fiber with the structure shown in FIG. 1 and a core size
D of about 30 .mu.m and t=750 nm. Bands are numbered from longest
wavelength band to shorter wavelength bands with increasing number.
As seen the hollow core PCF has four bands of low loss
transmission, wherein 3 of the low loss transmission bands comprise
wavelengths below 1.2 .mu.m.
[0140] The graph of FIG. 8 shows a closer view of a transmission
loss of the hollow core PCF of FIG. 1 with D=30 .mu.m and t=750 nm
of transmission band II. It can be seen that the low loss
transmission band is very broad with a bandwidth of 200-250 nm
around 1.064 .mu.m.
[0141] A simulation was made for a hollow core PCF with six hollow
tubes (prior art hollow core PCF) and with a hollow core PCF with
seven hollow tubes (an embodiment of the invention). The simulation
was done with D=30 .mu.m, t=750 nm, @1032 nm.
[0142] FIGS. 9 and 10 show HOM extinction ratio relative to d/D
(D=Dcore) of the prior art hollow core PCF with six hollow tubes
(FIG. 9) and the hollow core PCF of an embodiment of the invention
with seven hollow tubes (FIG. 10).
[0143] As it can be seen in FIG. 10 an optimal ratio d/D between
0.6 and 0.75 ensures that Lp11-like modes and several other HOMs of
higher order are resonantly coupled to cladding modes.
[0144] In FIG. 9 it can be seen that use of six hollow tubes allows
achieving a partly suppression of the Lp11-like modes. However,
modes with a higher azimuthal number remain unperturbed limiting
the overall HOM extinction.
[0145] Thus it is clearly shown that the hollow core PCF of the
invention with seven hollow tubes has much improved properties for
suppression of HOMs than the prior art hollow core PCF with six
hollow tubes.
[0146] Further comparison between hollow core PCF of an embodiment
of the invention and hollow core PCF having six hollow tubes are
disclosed in the article "Hollow-core fibers for high power pulse
delivery" by Michieletto et al. Optics Express 7103-7119, March
2016. The content of this article is hereby incorporated by
reference into this disclosure with the proviso that in case of any
inconsistence between the subject matter explicitly disclosed
herein and the incorporated subject matter, the content of the
subject matter explicitly disclosed herein prevail.
[0147] The laser system shown in FIG. 11 comprises a laser light
source 51 and a fiber delivery cable 52 for delivering light from
the laser light source 51 to a user apparatus 54. The fiber
delivery cable 52 comprises as its waveguide a hollow core PCF as
described above with one or more low loss transmission bands
correlated to the user apparatus. As indicated the fiber delivery
cable 52 may be rather long while still delivering single mode
light with high efficiency and low loss in the fundamental mode to
the user apparatus 54. The fiber delivery cable 52 has a first end
53a and a second end 53b. In the shown embodiment each of the first
end 53a and the second end 53b are mounted in a ferrule structure
for connecting respectively to the user apparatus 54 and the laser
light source 5.
[0148] In an alternative not shown embodiment the second end of the
fiber delivery cable 52 is spliced to a fiber output of the laser
light source 51.
[0149] The preform shown in FIG. 12 is a preform for an embodiment
of the hollow core fiber as described herein.
[0150] The preform comprises a preform outer cladding region 165
and 7 hollow preform tubes 161a, 161b arranged in a non-touching
ring (i.e. the tubes are not touching each other) surrounded by and
fused to the preform outer cladding region 165.
[0151] Pressure tubes 164 are arranged to connect each of three of
the preform tubes 161a (primary hollow tubes) to a not shown
pressure supply for controlling the pressure in the primary hollow
tubes 161a during drawing. A pressure tube length section 164a
inserted into the hole of each primary hollow tube 161a is
advantageously uncoated silica, whereas the remaining part of the
pressure tube 164, referred to as the pressure tube supply section
is polymer coated silica. The pressure in the secondary hollow
tubes 161b may advantageously be controlled in a pressure chamber
such as shown in FIGS. 15 and 16 of PA 2016 70262 DK with the title
"A RING ELEMENT FOR A PREFORM, A PREFORM AND AN OPTICAL FIBER DRAWN
FROM THE PREFORM".
[0152] In the embodiment shown in FIG. 13a the hollow core PCF
comprises an outer cladding region 171 and seven non-touching
hollow tubes 172 forming an inner cladding region and surrounded by
said outer cladding region 171. The hollow tubes 172 are fused at
fusing points 173 to the inner side of the outer cladding 171 to
form the inner cladding region and a hollow core region 174
surrounded by the inner cladding region. The outer cladding region
comprises a photonic band gap structure in the form of
microstructures (inclusions) 175a having different index than the
cladding background material 175b.
[0153] The photonic bandgap (PBG) structure may be provided by any
means, e.g. by providing the outer cladding region with an index
grating comprising concentric rings with different refractive index
and/or by including.
[0154] The inclusions 175a in the outer background material 175b
are arranged in a cross-sectional pattern comprising about 5
rings.
[0155] As mentioned it is preferred that the inclusions are voids
or of gas and have a relatively small diameter and are arranged
with short for optimizing (minimizing) confinement loss of the
desired wavelength or range of wavelength.
[0156] FIG. 13b shows the transmission loss for two variations of
the hollow core PCF of the embodiment FIG. 13a. wherein the two
variations of the hollow core PCF with PBG structure in their
respective cladding have been optimized for reducing the
confinement loss around a desired center wavelength of respectively
1064 nm (left) and 1550 nm (right). As it can be seen the loss is
very low. Further it is found that this approach to reduce the
confinement loss can be also be exploited in polarization
maintaining antiresonant fibers.
[0157] Further it was found that the confinement loss appears to be
arbitrary reduced by increasing the total thickness of the photonic
band gap structure.
Example 1
[0158] A hollow core PCF having a structure as shown in FIG. 1 was
fabricated using the stack and raw technique. The fabricated fiber
shown has a core diameter of approximately 30 .mu.m, d2.about.17
.mu.m with a d2/D.about.0.57. The mode field diameter measured at
1064 nm is 2 2 .mu.m. The tubes present minor size differences,
nonetheless the fiber shows remarkable low loss and bend loss and
good mode quality. The mode quality factor of the fabricated fiber
was measured the mode quality factor with a camera--based M.sup.2
measurement system (Spiricon M2-2OOS) with a laser at a wavelength
of 1064 nm and a 5 m FUT. We per-formed the measurements: with the
fiber coiled on a standard 8 cm spool and no further coils. The
results are summarized in Table I and shown in FIG. 14. The fiber
output beam presents negligible astigmatism and asymmetry and a
M.sup.2 of 1.2.
TABLE-US-00001 TABLE 1 Asymmetry Astigmatism M.sup.2 X M.sup.2 Y
1.02 0.01 1.22 1.2
* * * * *